This invention relates to well test apparatus and more particularly to annulus pressure operated test valves. Test valves are well known for testing the flow of fluid from a formation in a well hole. Test valves are also known for offshore testing from floating vessels.
Test valves are desired which can be remotely actuated to open and close a central passage and thereby control the passage of fluid from the formation up the central passage of a test string in which such a test valve is connected.
Various types of test valves for this purpose are known. One such test valve is one which is operated responsive to up and down mechanical movement of the test string. However, such a device is not suitable for testing offshore wells from a floating rig which is subjected to vertical motion.
Accordingly an alternate type of test valve has been developed which responds to annulus pressure within the annulus between the test valve and the well casing. An increase in annulus pressure causes the central passage valve to open and a decrease in annulus pressure causes the central passage valve to close. Hydrostatic annulus head pressure is used as a reference and only opens the test valve when the annulus pressure exceeds the reference. To this end it has been proposed to anticipate the magnitude of the hydrostatic pressure down hole and trap an equal amount of pressure in a pressure chamber. The trapped pressure is then used as a reference so that when annulus pressure exceeds the trapped pressure, the central passage valve opens. A differential control mechanism urges the valve closed when trapped pressure exceeds annulus pressure and urges the valve open when annulus pressure exceeds trapped pressure. This approach requires a relatively high pressure to be trapped in the test valve at the surface, which is dangerous, and requires a rather accurate precalculation of the down hole pressure.
To avoid trapping high pressure at the top of the well, test valves have been developed that have a pressure chamber which is open to annulus pressure as the test valve is lowered into the well. A mechanically operated pressure trapping valve is provided which is operated by mechanical down movement of the test string after the test valve has been positioned in place at the bottom of the well hole to thereby trap the hydrostatic annulus pressure in the pressure chamber as the reference.
One test valve is known which uses, in place of the mechanically operated pressure trapping valve, an annulus pressure operated pressure trapping valve. This device employs a differential pressure operated shuttle valve for trapping pressure in the pressure chamber. The differential pressure operated shuttle valve is spring biased open so that annulus pressure enters the pressure chamber. The force due to annulus pressure and central passage pressure acts in opposite directions on the shuttle valve and when annulus pressure is raised so that it exceeds central passage pressure, the shuttle valve closes thereby trapping annulus pressure in the pressure chamber. When annulus pressure decreases sufficiently, the shuttle valve reopens. This cycle is repeatable.
However, a problem may occur in such a test valve when a well is acidized through the test valve. When acidizing, annulus pressure is first increased, first causing the shuttle valve to close, trapping approximately hydrostatic annulus pressure and second, causing the differential control for the central passage valve to open the central passage valve. Pressurized acid is pumped down the tubing in which the test valve is connected. However, should the elevated pressure of the acid in the central passage in combination with spring force cause the shuttle valve to open, the pressure will suddenly increase in the pressure chamber, eliminating the pressure differential which is utilized by the differential control for maintaining the central passage valve open. This will cause the test valve to malfunction and erroneously close the central passage valve.